Public High School Debate: Upgrade or Repair?

By Consulting Specifying Engineer Staff

12/21/2004

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"For more than 25 years, we relied upon two huge cast iron boilers to get us through the harsh New England winters," said Gary Persechetti, Supervisor of Building and Grounds for Chelmsford High School. "When they gave out, it would have cost more to repair them than to replace them. A contractor from one of our construction projects introduced us to AERCO, who demonstrated the benefits of a modulating and condensing boiler."

Persechetti was impressed by the demonstration, and in 2000 ordered five gas-fired Benchmark units from AERCO to support the school. Not only did the 10 million BTU/hr. boiler plant occupy less space than just one of the cast iron boilers, AERCO's condensing design made it possible to eliminate all secondary pumps and associated controls, wiring, piping and valves, which streamlined the school's overall heating system.

By extracting latent energy from water vapors created during combustion, AERCO's condensing design proved to be significantly more energy efficient than the conventional equipment. AERCO's Boiler Management System (BMS) coordinates the operation of the five Benchmark units. It provides each unit with firing instructions that ensure the plant's maximum efficiency as it deals with fluctuating loads throughout the day. The plant operates on the simple principle that it requires less energy for a group of modulating boilers, each firing at "part-load" to heat a building, than a single boiler operating at "full fire" to carry the entire workload. The ability to operate the AERCO boilers at lower water temperatures allowed Chelmsford High School to implement an indoor/outdoor temperature reset schedule to further enhance the system's efficiency.

The Bottom Line - $81,000 in Energy SavingsEnergy costs at the high school have decreased significantly since the AERCO system was implemented. During the first year of operation, 2000-2001, Chelmsford reduced fuel consumption by 33% from the previous year.

"Our lower fuel consumption saved us $81,000," said Persechetti. "Because of the results that AERCO showed in their first year at Chelmsford High School, we've installed Benchmarks at three additional schools. They're everything you want in a boiler - efficient and self-reliant."

Indiana Middle School Phases Out Steam

Like many older schools, Geyer Middle School in Fort Wayne, Indiana was heated by two large steam boilers. In 2001, the school embarked on a project to replace these 42 year-old units with high-efficiency AERCO equipment.

"We initially installed two, 2 million BTU/hr. Benchmark units to handle the 118,084 sq. ft. building," said Stephen Novell, supervisor of mechanical maintenance at the Fort Wayne Community School System. "We kept one of the old boilers because a portion of the building remained on a steam system. In 2002 we pulled the last steam boiler and converted the remainder of the building to hot water. At this point we also installed a third Benchmark unit. It provided the capacity cushion we wanted, while adding to the efficiency profile of the overall heating plant."

Because the Benchmark is designed to condense, Novell simplified the school's heating system. The era of mixing valves, hot water mixing loops and temperature averaging components to protect the boilers from thermal shock was over. All of this became unnecessary with the efficient Benchmark's condensing capabilities - the process of extracting latent energy from water vapors created during the combustion process and releasing it as useable heat.

Just as important to thermal efficiency is the Benchmark's modulating design, which precisely matches the boiler's firing rate to actual building demand. Rather than incur burner cycling losses and wasteful overshooting, which stems from a strictly "on/off" burner operation characterized by the old boilers, each 2 million BTU/hr. Benchmark can run with as little as 100,000 BTU/hr. - or 5% input. The Benchmark can gradually increase capacity, as needed - in precise 1% increments - until it's firing at 100%. In fact, the lower the firing rate, the higher the Benchmark's efficiency.

To fully leverage this capability, the three Benchmark units function as a single system under AERCO's Boiler Management System (BMS). The BMS regulates the units to operate in tandem, providing them with precise firing instructions that ensure maximum efficiency. It is more efficient for the three boilers to run at 30% input than for two to run at 45%, or for one boiler to run at 90% capacity. The BMS also consistently rotates the lead boiler in the plant so that each unit receives approximately the same amount of work over the course of the season.

"We've achieved a massive decrease in natural gas consumption since switching to AERCO," said Novell. "They run great, they save us money and, because we haven't had any maintenance problems, we barely notice they're there. That's exactly what someone wants in a heating system."

Bottom Line - 29.43% Drop in Energy ConsumptionWhen comparing energy consumption of the steam plant in the winter of 2001 to the more severe winter of 2003 with the Benchmark plant fully in place, energy consumption decreased by 29.43%.